Field of Invention
The present invention relates to a probe for monitoring the surface level of a fluid in a vessel, and to a method of installing the apparatus in the vessel. The apparatus of the present invention is applicable in all such vessels, containers, tanks, drop legs, blow tanks, reactors, etc. where information on the surface level of the fluid present in the vessel is needed for controlling the application. An example is a vessel from which the fluid is about to be pumped further such that the surface level in the vessel is maintained within certain limits, whereby the operation of the feed and/or discharge pump have/has to be adjusted in relation to the surface level in the vessel.
Background Information
The prior art discusses dozens of ways for monitoring the surface level of a fluid in a vessel.
Pressure sensors may be used in cases where the fluid in the vessel is an easily flowable one, like water, and where the diameter of the vessel is substantially large. In other words, pressure sensors are applicable only in cases where the pressure value may be measured in a reliable and reproducible manner.
Capacitive sensors or wires are widely used, but they are very sensitive to the consistency or dry material content of the fluid. They also collect dirt and lose their sensitivity fairly quickly. Additionally, they, especially the wires are mechanically weak, i.e. they may loosen or break, and, at worst, end up in the fluid and along with the fluid in the pump and in the further process.
As an example of documents discussing capacitive sensors may be mentioned in US-A-2012/0184932, which discusses a drainage pump unit for aspirating fluids by a suction pump arranged in a suction pump housing, wherein a fluid collection container can be secured releasably on the suction pump housing, and wherein a capacitive filling level sensor is arranged on the suction pump housing for detecting a filling level in the fluid collection container. The filling level sensor has at least two electrodes, which are arranged at a distance from each other and extend along a common path. A first of the electrodes is in one piece, and a second of the electrodes is segmented. Here the fluid collection container has a rectangular cross section and the electrodes are arranged directly on one of the planar side walls of the container.
US-A1-2010/0251816 discusses an aircraft fuel measurement system that utilizes one or more digital probes that is/are arranged inside the fuel tank. The probes are formed of two concentric tubes arranged at a certain distance from one another and includes brackets for mounting the probe/s inside the fuel tank.
JP-A-2005283201 discusses yet another capacitive system for monitoring the liquid level in a reducing agent container, the reducing agent being used in an emission control device of a motor vehicle. The probe is formed from two concentric tubes arranged at a certain distance from one another and includes a bracket for mounting the probe via an opening in the top cover of the container inside the urea container.
WO-A1-03052357 discusses a filling level sensor comprising an electrode assembly with several individual electrodes and a backplate electrode common to the electrodes disposed outside the vessel within a hose pipe, which preferably extends over the entire height of said vessel and the ends of which are flow-connected respectively to a tapping connector and a feed connector of the vessel. In other words, the electrode arrangement is arranged outside the vessel in a separate pipe extending over the entire height of the vessel so that since the pipe is in flow communication with the vessel the liquid level in the pipe is the same as that in the vessel, as long as the liquid is easily flowable.
US-A1-2005/0242966 discusses a system for detecting the overflow of a liquid from a vessel, such as a bathtub. The vessel wall includes pairs of sensors, which may be electrical, measuring resistance, or optical, detecting an obstructed line of sight.
U.S. Pat. No. 5,167,769 discusses a sensor for the detection of wood chip level in a digester, for example, a Kamyr digester, which consists of: (a) a probe, several meters long, vertically mounted inside the top part of the digester; (b) a series of bare metal electrodes in the side of the probe, spaced typically a few centimeters apart, and electrically insulated from the probe wall; (c) electronic circuit processing to allow any pair of electrodes to be selected, and (c) a device (or means) to allow the electrical properties of a liquor or wood chip and liquor mixture between the electrodes of a pair to be measured, and the level deduced from the measurement. Here the probe has been arranged along the vertical centerline of the digester.
U.S. Pat. No. 3,313,902 discusses controlling of pumping an electrically conductive liquid to a tank by an electrode arranged angularly inside the tank. The wall of the tank functions as the second electrode. When installing the electrode in the tank, the tank wall first includes an opening, and the opening includes a sleeve-like coupling member, through which the electrode may be slipped inside the tank. After the electrode is in its inclined position in the tank a seal ring and another sleeve like coupling member is slid over the electrode whereafter the two coupling members are tightened to one another.
Radar-based devices have also been experimented, but it has been learned that they are sensitive to steam, to the topography of the surface and to various structures inside the vessel. Additionally, the construction and size of the radar-based devices set their own demands on the positioning of the device in the vessel such that it does not interfere into the internal flows of fluid in the vessel.
Radiometric measurements have their own downsides, i.e. limited acceptance due to the use of the radiation source, limited operational time, required permits as well as other specific regulations.
As discussed above, all devices used, in one way or another, for measuring a surface level of a fluid in a vessel have their own problems and downsides. There are additional problems when the fluid is not a liquid that behaves like water. The fluid may have a consistency, i.e. dry matter content, of, for instance, between 8 and 40%. The fluid may be lumpy whereby the surface topography is very uneven. The fluid may be hot whereby steam evaporates out of the fluid. The fluid may be hardly flowable whereby 1) there is no clear surface in the vessel, 2) it is out of the question to arrange any internal structures in the vessel such that they are located at a considerable distance from the wall of the vessel, i.e. at the main downward flow area in the vessel, as such structures would interfere into the internal flows of fluid in the vessel and/or would be subject to breakage when possibly lumpy material drops on or tends to bend such structures, and 3) the fluid does not create any pressure that could be used in monitoring the surface level of the fluid.
A yet further problem in the prior art devices or apparatus used in monitoring the surface level in a vessel is that they are either devices that have been integrated into a structure, for instance a wall, of the vessel, or they may be installed or repaired only by entering the interior of the vessel. The former type of device is mainly in use in very small apparatuses, where, in case of malfunction, the entire wall of the apparatus or the entire apparatus may be changed. Retrofitting of the latter type of apparatus in substantially small vessels (no entrance for installation personnel) or maintenance of apparatus in such devices has not, this far, been paid any attention to. Yet another problem in some prior art devices is that the probe is arranged to be installed in the vessel via an opening in the top cover of the vessel. If the height of the vessel is several meters and the level of the fluid surface is supposed to fluctuate somewhere in the middle part of the vessel the required length of the probe makes the probe very vulnerable unless it is built sturdy enough. Another problem relating to the probe arranged to the top cover of the vessel may be seen in vessels where there is, in practice, no top cover at all, but the vessel is formed of a bend at the top thereof and a downflow pipe both having the same constant diameter. In such a case, the flow in the vessel takes place over the entire cross section of the vessel, and would subject a top arranged probe to considerable stress.
The above problems contain such multitude or variety of requirements relating to strength and varying process conditions that preclude that the vessel has to be manufactured of metal, which is still the preferred material for the most demanding operating environments.